CHAPTER XXXXVI.

Section 254. The Subject of +depressing torrents+ requires an accurate Investigation: as it will serve to point out the proper Time of Day or Night, when an Aironaut ought so to calculate his Voyage, as to arrive over the Middle of the Channel, or Arm of the Sea, at some particular Hour: in order to wait for a Sea Breeze which may waft him to the other Side.

A Point not difficult to be ascertained.

Also, this Idea of +depression+, if properly considered and digested; may prove a sufficient Foundation on which to establish a new Theory of the _Weather_, so ill determined at present, from its _aggregate Weight_ or _Elasticity_ only, as indicated by the Barometer.

255. If a Conjecture may be formed on a Subject, material in itself, yet of which so little is actually known; woud not _the proper Time_ of undertaking a Voyage over the Channel be such, that the Aironaut shoud find himself three Parts of the Way across, by +nine+ _o’Clock_ in the Morning?

256. In _warmer_ Climates, where the Seasons are more regular; the _Land-Breeze_ blows to Sea from Midnight till X. in the Morning: at which Time, the _Sea-Breeze_ blows to Land; continues till V. or VI. in the Evening; and is succeeded by a +calm+, which lasts till Midnight.

Whence it follows, that during the Time of the Sea-Breeze, there is a constant Tendency towards a +gulph of air+, _along the Middle_ of the Channel: the Equilibrium of which is as constantly supplied by a _Depression_ of the upper and in general cooler Strata of Air; and therefore a _dangerous_ Time for the Passage of Balloons.

On the contrary, during the Night, and till ten in the Morning, there is an _Accumulation_ of Air, _along the Middle of the Channel_: which consequently is a proper Time to ensure a _safe_ Passage; by the Assistance of +wings+, or some +propulsive+ Machinery.

[Sidenote: Of the horizontally calm mediocèanal depressing Current.]

257. The Deficiency or Vacuity being supplied from the etherial Regions; it might be taken for granted, that such Ether must be _considerably_ lighter than the adjacent common Air on an equal Level, and therefore _proportionably_ dangerous for the Passage of Balloons.

But if it be considered that such Air, acting as a +wedge+, or more probably in the Form of an hyperbòlic Solid,[68] to fill up the Vacuity, descends with Rapidity from a _colder_ Atmosphere impregnated with aqueous Vapours _invisible from below_; and that both the Air and Vapour have reciprocal Affinities and Attractions, electric and mechanical, with the Body of Water beneath them; and are often rendered still cooler by its constant Agitation and _Evaporation_; also, that the Supply being immediate and cotemporary, with the +double tide of air+ flowing from the _middle_ over the _opposite_ Shores;—there possibly may be little or no Difference between the aggregate or _barometric_ Gravity of _such_ Columns, and those which are formed by the Sea-Breeze on either Side of them: therefore the Descent of Balloons is owing, among other Causes, to an almost perpendicular actual Depression of the superincumbent Atmosphere.[69]

Following up the Idea of a Sea-Breeze, blowing, at a Medium, for 20 Miles over Land; altho’ the Stratum of the +lower current+ of Air, or Sea Breeze, may not exceed +half a mile in depth+, measuring from the Ground upwards; nearly equal to 26 Inches of the Barometer _above_, the Thermometer also _above_ being at 55, i.e. _Temperate_:—yet this Observation may prove of essential Service, while the +upper current+ of Air, i.e. the general Wind blows +towards+ the Sea, (which will be found to take Place more _frequently_ than is, at present, imagined;) or while the Balloon is influenced that Way; as was the Case with Sadler and his Companion when over the Nore: who, on his accidental and sudden Descent, fortunately found Safety in the +sea-breeze+.

Which Breeze was sought for, and made Use of by the Author, when in the Balloon, near Frodsham, in Cheshire.

For, as the Sea-Breeze is pretty general, Aironauts shoud not be too apprehensive: as they have it in their Power, by proper Management, to drop into the Breeze—for +either shore+: if they are provided with a Machinery to waft themselves across the intermediate _depressing_ or _accumulating_ +mediocèanal column of air+: which Space, between the two Shores, is, as before hinted, frequently +becalmed+.

258. Further: as the above Theory of a _mediocèanal_ Depression seems to receive additional Confirmation from _each_ Balloon Experiment; Lunardi _descending_ on the 5th of October last, when near the Middle of the Bay of Edinburgh or Firth of Forth;—it may be found _prudent_, to keep the Balloon continually rising, till the Aironaut is _one-third_ of the Passage _over_.

258. 2. For if the general Wind in the upper Current be not strong; the Aironaut may expect to be _becalmed_, with Respect to the horizontal Direction of the Current, the Instant he finds, by the Rise of the Barometer, that the Balloon _descends_; i.e. when it is acted upon by the depressing Column: in which Case, the _higher_ he has soared, the _safer_: as he will have more Room and greater _Latitude_ for Exertion by Means of the Machinery: which Machinery will be greatly _aided_ by the Force of the descending Column or Gravity; and will act on a similar Principle with the Ferry-Boats over the River Po in Italy; which are a Sort of horizontal Pendulum. For the Aironauts will continue to _descend_, at the same Time that their _Wings_ furnish the Means of a progressive Motion.

Therefore, before the Time that the Balloon has reached the Surface of the Water; they will have crossed the depressing Column; and find themselves wafted _gently_ by the _new_ Sea-Breeze setting in towards the opposite Shore.

259. If the Aironaut _rises up_ to Sea with a Wind blowing from the Land on each of the opposite Sides of the Channel, and arrives above the Middle of the Channel, while the same Wind remains; it is probable that the Balloon will continue to rise higher as he proceeds towards the Middle, _where_ the +mediocèanal accumulation+ has for some Hours taken Place; and therefore he need not be under any Apprehension of falling: but, as before, it being probable he will also be _becalmed_; the Necessity of propulsive Machinery is equally urgent, in order to pass the Center of the _Accumulation_: after which, the Balloon will ride Home to the opposite Shore in the new Sea-Breeze, by _that_ Time, just beginning to set in.

260. With the Assistance of propulsive Machinery, it is imagined the Aironaut may be enabled in a few Minutes to force throu’ the calm mediocèanal Accumulation, or Depression: after which, he will have little Occasion to make Use of it.

261. +Sunrise+ is, probably, the +safest+ Time of all, to ascend towards the Sea, with an _Air-tight_ Balloon: arriving with the Assistance of the Wings, throu’ the _calm_ mediocèanal Accumulation: and there waiting till the new _Sea-Breeze_ sets in to the _opposite_ Shore.